This invention relates to the three-wire method of thread pitch diameter measurement, and it is more particularly concerned with a new and improved apparatus for use in practice of this method.
The three-wire method of thread pitch diameter measurement is a known procedure used to measure the pitch diameter of a thread, for example during machining of a thread. The method comprises the use of a set of three wires of uniform circular cross section which are of the same known diameter. Two wires are positioned on one side of the pitch diameter being measured and disposed in immediately adjacent convolutions of the thread root at the same circumferential point about the thread axis while the other wire is disposed on the exact diametrically opposite side in the portion of the root convolution which is between the first two wires. The wires do not seat fully within the roots but rather each wire is disposed against a flank of the immediately adjacent crest convolutions which lie on each side of the root convolution.
According to the theory of three line pitch diameter measurement, an accurate pitch diameter measurement can be obtained across the three wires at their points of tangency with the thread if the points of tangency of the two wires on one side are exactly the same and that of the single wire on the opposite side is exactly 180.degree. from the points of tangency of the first two wires. Through knowledge of the diameters of the wires and a micrometer measurement across the three wires, the pitch diameter of the thread may be calculated.
In order to obtain an accurate measurement, it is important that the wires be located in the manner explained above. Manual procedures for manipulating the wires and holding them in the desired positions for accurate measurement are difficult. Hence, it may not be possible to obtain consistently accurate measurements unless one is very skilled in the procedure, and therefore, various types of apparatus for use in holding and manipulating the wires have heretofore been proposed. Examples of these are illustrated in the following U.S. patents which were developed in connection with a novelty search conducted relative to this invention.
U.S. Pat. No. 2,554,024 PA1 U.S. Pat. No. 2,663,948 PA1 U.S. Pat. No. 2,692,438 PA1 U.S. Pat. No. 2,741,035 PA1 U.S. Pat. No. 2,939,220 PA1 U.S. Pat. No. 2,962,817 PA1 U.S. Pat. No. 3,068,582 PA1 U.S. Pat. No. 3,253,340
U.S. Pat. No. 3,253,340 comprises a mechanism which resiliently holds the wires by means of jaw-like elements and which is adjustable while maintaining the held wires in parallel planes.
U.S. Pat. No. 2,663,948 illustrates another type of holder which maintains the held wires parallel.
U.S. Pat. No. 2,554,024 illustrates another type of wire holder with adjustment capabilities.
U.S. Pat. No. 3,068,582 illustrates a holder containing grooves for locating the wires.
The present invention is directed to a new and improved apparatus for use with the three-wire method of thread pitch diameter measurement. In particular, it comprises an arrangement and construction which facilitates the mounting of the wires and their accurate application to a thread whose pitch diameter is to be measured.
One important feature of the invention is that it does not have a complicated construction, yet it is capable of quickly and accurately locating wires for use in the three-wire method so that an accurate measurement can be quickly obtained. Another feature is that any given embodiment of the invention can accommodate a number of wires of varying diameters and hence it is unnecessary to have an individual embodiment for each three-wire set. Moreover, the construction is such that varying diameters and pitches can be measured with a given embodiment of the invention. The wires can be readily inserted and changed and the apparatus adjusted as required for different measurements.
Briefly, the preferred embodiment of the invention comprises a pair of holder elements which are guided for relative straight line movement toward and away from each other in the direction of the measured diameter. The holder elements, which are of a generally rectangular shape, comprise wire-engaging surfaces which are parallel and confront each other. One wire is disposed against the wire-engaging surface of the one holder element and the other two wires against the wire-engaging surface of the other holder element. Retention of the wires on the holder elements is accomplished by means of rubber bands. The holder elements also define locating slots which generally locate the wire elements so that they will be generally tangent to the fastener, yet the construction is such that different thread sizes may be measured with the same holder and different wire diameters may also be used. The two holder elements are resiliently urged together to urge the wires against the opposite sides of the thread pitch diameter being measured while ensuring that the wires on opposite sides are disposed in parallel planes. Circular apertures extend through the two holder elements, passing through the wire-engaging surfaces thereof, to provide access to the three wires at their points of tangency with the thread being measured so that a measuring instrument may be engaged via the apertures with the three wires at their points of tangency with the thread to obtain an accurate measurement.
The foregoing features, advantages and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.